1. Field of the Invention
The present invention relates to the inspection by radiography of an object possessing a crystal lattice.
2. Discussion of the Background
Non-destructive tests (also called non-destructive testing) make it possible to provide information about the condition of a part or a structure without any resulting changes which could prejudice their ulterior use. Thus it is possible to detect internal defects which cannot be revealed during a superficial inspection. Internal defects in a part can vary greatly: shape defects, dimension defects, the presence of inclusions, cracks, etc.
The principle of detecting a defect, according to non-destructive testing, consists of exciting the defect and collecting its response. Thus a part to be inspected can be submitted to a flux of ionising radiation and, by studying the transmitted flux, one can detect an eventual defect thanks to the perturbation of the flux it introduces.
By submitting the part to be inspected to very short wavelength electromagnetic radiation (X or .gamma.), it is possible to collect intensity modulations of the beam transmitted under the form of an image on an appropriate receptor (for example a film). The radiography (X or .gamma.) uses a detector in real time which is an element or an assembly of elements transforming photons (X or .gamma.) into an electric signal which at the outlet delivers either an analogue signal, or digital data. Besides these X and .gamma. radiography procedures, there is also neutrography which uses neutron beams.
According to the radioscopy (X or .gamma.) procedure the radiographic image is captured, by a fluorescent screen for example, and produces an optical image which can be observed directly and in real time.
Contrary to a simple radiography, tomography makes it possible to provide information about the complete morphology of a defect thanks to several exposures taken under different angles.
The field of application of industrial radiography is very vast and concerns parts of all kinds.
However, when an exposure is made under the classic conditions of radiography or radioscopy, if the part being inspected possesses a crystal lattice, this irradiated crystal lattice emits radiations of the same wavelength as the electromagnetic radiation crossing through it. This secondary radiation provokes diffraction according to the Bragg equation. This diffraction causes parasitic radiation, creating confusion between the defects present in the part and the parasitic indications. These parasitic radiations can also mask the indications being looked for.